What are the eye signs of Horner's syndrome?

Eye Signs of Horner's Syndrome

The classic triad of Horner's syndrome consists of ipsilateral ptosis, miosis (smaller pupil on the affected side), and facial anhidrosis, all resulting from interruption of the oculosympathetic pathway. 1, 2

Core Clinical Features

Pupillary Findings

• Miosis: The affected pupil is smaller but remains normally reactive to light 1, 2
• Anisocoria: Pupillary asymmetry that is more pronounced in dim lighting or darkness 3
• The pupil constricts briskly to light without an afferent pupillary defect 3
• Anisocoria greater than 1 millimeter warrants investigation for pathological processes including Horner syndrome 4

Eyelid Abnormalities

• Mild ptosis (blepharoptosis) of the upper eyelid on the affected side 1, 3, 2
• The ptosis is typically subtle, presenting as a "slightly droopy upper lid" 2
• Levator function assessment should be performed as part of the external examination 4

Facial Changes

• Facial anhidrosis (decreased sweating) on the ipsilateral side, though this is often not prominent 5
• In postganglionic Horner syndrome, anhidrosis may be virtually absent 5
• When present, anhidrosis may be found on the medial side of the forehead 3

Associated Neuro-Ophthalmic Signs

When evaluating for Horner syndrome, carefully check for other neuro-ophthalmic signs that may indicate the underlying etiology and localization:

Brainstem/Central Lesions

• Cranial nerve palsies (III, IV, VI) 4, 6
• Internuclear ophthalmoplegia (INO) 4
• Nystagmus 4
• Hemiparesis or sensory loss 4
• Ataxia 4, 6

Cavernous Sinus Involvement

• Concurrent third, fourth, or sixth nerve palsies may occur when sympathetic fibers are damaged in the cavernous sinus 6

Skull Base/Trigeminal Involvement

• Hypoesthesia in cranial nerve V territories (V1-3) 3
• Ipsilateral headache or facial pain 3, 5

Key Diagnostic Considerations

Pharmacologic Confirmation

• 10% cocaine test can confirm the presence of Horner syndrome 3
• Apraclonidine appears to be a good alternative to cocaine for confirming the diagnosis 5
• Pharmacologic testing can both confirm presence and direct further management 1

Critical Assessment Approach

• Immediately assess pupil symmetry: bilateral miosis suggests toxic/metabolic cause or pontine pathology, while unilateral suggests Horner syndrome 7
• Evaluate light reactivity, which is preserved in Horner syndrome 7
• Assess associated neurologic signs including extraocular movements and extremity strength 7

Common Pitfalls

Do not confuse anisocoria with abnormal pupillary light response—these represent different pathophysiologic processes 7. In Horner syndrome, the pupil remains reactive to light despite being smaller.

Anhidrosis is rarely prominent and should not be relied upon as a primary diagnostic feature, particularly in postganglionic lesions 5. The pupillary and eyelid findings are more reliable.

The condition does not cause vision problems or significant visual symptoms—it serves primarily as a warning sign of oculosympathetic pathway interruption 2.